Superheterodyne receiver with automatic frequency control



Oct. 4, 1949:

F A. DE GROOT 2,483,889

SUPERHETERODYNE RECEIVER WITH AUTOMATIC FREQUENCY CONTROL Filed April12, 1946 FOLKERTALBERZ'DE 62001" IN VEN TOR.

ATTORNEY.

Patented Oct. 4, 1949 UNITED sTA r rem OFFICE SUPERHETERODYNE RECEIVERWITH AUTOMATIC FREQUENCY CONTROL Conn, as trustee Application April 12,1946', S'erial'No. 661,521

In the Netherlands April 16, 1943 7 Claims.

The disclosure of U. S. Patent application Serial No. 661,523, filedApril 12, 1946, relates to a superheterodyne receiver whoseintermediatefrequency portion comprises a sharp filter for the selectionof the carrier wave and wherein a control voltageforthe automaticfrequency control is generated with the aid of afrequency-responsive-network to which areconnected one or more controlrectifiers whilst exclusively owing to the influence exerted bythe saidcontrol voltage on the tuning of the sharp filter theintermediatefrequencycarrier wave is kept within the frequency rangeselected by this filter and the filter, which comprises one ormore-oscillatory circuits with reduced damping, forms at the same timepart of the frequency-responsive network.

The damping of the oscillatory circuits is generally reduced with theaidof a back-coupled discharge system or by means of-a discharge systemwherein a negative resistance occurs between two electrodes.

By selection is construed herein to include the accentuation of thecarrier wave with respect tothe side-band frequencies as well as theseparation or suppression of the carrier wave.

With a receiver of this type it is possible, for

example, to. eliminate the serious distortion which frequently occursdue to. the fact that the carrier wave of the received signal isattenuated to ahigher extent than are the sidebands (selective fadingeffect). This is achieved by utilizing in the receiving channel a sharpfilter which; accentuates the carrier wave with respect to theside-bands. influenced-by. the control: voltage of the-antimmaticfrequency control in such manner that the intermediate-frequency carrierwave is; kept within thefrequency range selected by the filter.

In. orderv to. be able to vary the tuning ofthe sharp. filter over acomparativelywide range, use is preferably made of a filter whichcomprises one or more oscillatory circuits with reduced damping.

The present invention relates to one determined form of construction of'the above-de-- scribed superheterodyne receiver, viz. to a receiverprovided with a sharp filter which com-- prises: at least one inductancecoil provided with a ferromagnetic core (iron-cored coil) whoseself-induction is influenced by the automatic frequency. control. inorder to be able-to give the The tuning of this: filter is (Cl.179--IE71)- cored coils in the above-described manner entails drawbackssince the circuit-arrangement either has. no or little filtering efiector begins to oscillate. According to the invention, these drawbacks areavoided by incorporating the ironcored coil in an oscillatory circuitwith reduced damping in such manner that variations in the qualityfactor, or Q, of this circuit, due to the effect of the automaticfrequency control on the iron cored coil, have no harmful effect on thestability of the circuit with reduced damping or on: the selectivity ofthe filter.

For, due to the elfect of the automatic frequency control, it is notonly the selfinduction but in general also the Q of the iron-cored coiland the: Q ofthecircuit that are modified. Since, however, inorder toensure a satisfactory functioning of the filter, it is necessary toreduce the damping of the'circuit to a high extent, the Q of the circuitmust not exhibit great variations; since even comparatively smallvariations of the Q exert a very great influence on the circuit andconsequently on the functioning of the filter.

In fact, if owing to the effect of the automatic frequency control onthe iron-cored coil the Q of the circuit is improved, the latterapproaches very soonthe point at which self-oscillation occurs. If, onthe contrary, the Q of the circuit decreases, the selectivity of thefilter may be reduced to such an extent that there is no longer anyquestion of selection of the carrier wave, so that in both cases theworking of the circuitarrangementdoes not appear to full advantage. Bycarrying the invention into effect, the abovementioned-drawbacks areavoided.

The iron-cored coil is preferably incorporated in the oscillatorycircuit with reduced damping in such manner that within the tuning rangeof the filter the Q- of the said circuit is not or slightly modified.

If the oscillatory circuit with reduced damping comprises bothanair-coil and an iron-cored coil and if the reduction of the damping isbrought about with the aid. of a back-coupled discharge system, this maybe achieved by incorporating the whole or part of the iron-cored coilinthe back-coupling circuit of the discharge system by means of whichthe damping of l the circuit is reduced.

If in the oscillatory circuit with reduced damping use'is made of aniron-cored coil whose core material and/or whose dimensions have been.sochosen that upon variation of the selfinduction of this coil theseriesresistance remains constant or substantially constant, we also ob-' taina circuit whose Q does not vary or slightly varies.

It is in general advantageous negatively to couple back thedamping-reducing discharge system, and this in such manner that thecircuit in itself is not damped by the negative backcoupling. By takingthis step it is avoided that in the case of a strong reduction of thedamping of the filter there occurs self-oscillation due to variations ofthe quantities which determine the behaviour of the damping-reducingdischarge system.

The invention will be explained more fully with reference to theaccompanying drawing, which represents, by way of example, oneembodiment thereof.

The drawing represents the: intermediate-frequency portion of a receiverto which the invention has been applied. An intermediate-frequencyamplifying tube I is coupled to the control grid circuit of a dischargetube A through the intermediary of an oscillatory circuit 2, which istuned to the intermediate-frequency, and of a filter 3, which isconstituted by a circuit with reduced damping. The filter 3 is composedof a condenser 5, an iron-cored coil 6 and an air-core coil 7. Thereduction of the damping of the circuit 3 is brought about owing to thefact that the circuit 3 is connected in three-point connection :1

to the discharge tube A; that is, circuit 3 is connected in regenerativefeedback relationship to the tube 4. Moreover, the discharge tube l isin negative feedback relationship since a resistance 8 which is notshunted for the intermediatefrequency is connected in the cathode lead.The filter 3 is not damped by the presence of this resistance.

In the anode circuit of the discharge tube 4 is incorporated a circuit 9which is tuned to the intermediate frequency and which is capacitativelycoupled, via a condenser ID, to a diode-detector H' whose low-frequencyoutput voltage is supplied to a low-frequency amplifier (not shown).

Furthermore, the circuit 2 has coupled to it a circuit [2 which is alsotuned to the intermediatefrequency and to which may be connected twopush-pull control rectifiers (diodes) l3 and It. The mid-point of thecircuit l2 and the mid-point of the output resistance l5 of the twodiodes l3 and Id are connected to a coupling coil i6 which isinductively coupled to the intermediate-frequency circuit 9. Thecircuits 1, 9 and I2 and the coupling coil i6 form afrequency-responsive network which, jointly with the control rectifiersl3 and I4 connected thereto, forms a device for generating a controlvoltage for automatic frequency control, which control voltage appearsacross the above-mentioned output resistance P5.

The control voltage is supplied to the control grid of a discharge tubeH which is shown as a triode and whose anode current flows through amagnetizing coil [8 of a transformer [9 which comprises a core ofhigh-frequency iron on which a magnetizing coil I8 and the previouslydescribed coil 6 are arranged. By varying the direct current flowingthrough this coil, it is possible to vary the inductance of the coil 6and therefore the tuning of the sharp circuit 3 within narrow limits.

Due to the presence of the sharp circuit 3 the side-band frequencies ofthe intermediate-frequency signal are greatly attenuated with respect tothe carrier wave, owing to which in the case of selective fading thedanger of the production of an apparent over-modulation of the signal,which would be attended with a large non-linear distortion of thelow-frequency signal, is avoided. The linear distortion caused by theattenuation of the side-band frequencies is suppressed by the properchoice of the frequency characteristic of the low-frequency amplifier.

In order to obtain the desired effect, it is necessary, however, thatthe tuning frequency of the circuit should always correspond withinnarrow limits to the frequency of the intermediate-frequency carrierwave. This correspondence is ensured by the automatic frequency control,for

; when there occurs a difference between the said frequencies there isset up across the resistance I5 a control voltage of suitable polaritywhich voltage is applied to the input circuit of tube 11, the anodecurrent of which flows through coil l8 and thus modifies the inductancevalue of the coil 6 to such an extent that the tuning frequency of thecircuit 3 approximately corresponds again to the carrier wave frequency.

Due to the effect of the automatic frequency control it is now not onlythe selfinduction but in general also the Q of the iron-cored coil 6 andof the circuit 3 which are modified. Besides, dependently on the kind ofiron-cored coil, it may occur not only that the Q of the circuit 3increases but also that the Q decreases at an increasing value of theselfinduction. These Q variations may be of such a value that thesatisfactory func tioning of the receiver is endangered or even becomesquite impossible, for improvement of the Q may result in that thedamping of the circuit is completely eliminated and that the circuitstarts oscillating. This phenomenon readily occurs in practice since acomparatively slight improvement of the Q exerts a comparatively greatinfluence upon the degree of the reduction of the damping of the circuitwhose damping is already greatly reduced.

If, on the contrary, the Q gets worse the selectivity of the filterdecreases, and this to such an I extent that a comparatively slightdecrease of the Q of the circuit results in a comparatively largedecrease of the selectivity of the filter. If, however, the filter doesno longer function selectively, it is not only the carrier wave but alsoan appre- Z ciable portion of the side bands that is selected,

due to which a new distortion is introduced and the distortion due toselective fading is no longer suppressed.

According to the invention, for this reason the iron-cored coil 6 isincorporated in the oscillatory circuit with reduced damping in suchmanner that within the tuning range of the filter the Q of this circuitdoes not vary or slightly varies. Here this is achieved by incorporatingpart of the iron-cored coil in the back-coupling circuit of thedischarge tube 4. The anode circuit of this tube is connected for thispurpose to a tap on the iron-cored coil 6.

It is also possible, however, to incorporate the whole of the iron-coredcoil in the anode circuit if in this case the discharge system 4 isconnected to properly chosen taps on the air coil I.

If now, due to the effect of the automatic frequency control, the tuningof the filter 3 is modified, there does not longer arise the risk thatthe selectivity of the filter becomes excessively low or that thecircuit starts oscillating.

What I claim is:

1 An amplifier circuit arrangement, particuto couple said resonantcircuit and said filtercir cuit in cascade to the input circuit of saidamplifier to' apply asignal voltage of frequency corresponding to saidcarrier and sideband frequencies tosaidamplifier, means to couple saidfilter circuit, tosaid amplifier to derive and apply to the inputcircuitof said amplifier a second'signal voltageof frequency corresponding tosaid carrier frequency to reduce the damping of said filter, means,coupled to said resonant circuit and the output circuit of saidamplifier to produce a frequency control voltage, means responsive tosaid control voltage to vary the magnetization of said ferromagneticcore to tune said filter sharply to an intermediate frequencycorresponding to said carrier frequency,means coupling the outputcircuit of said amplifier to said filter circuit in regenerativefeedback relationship to maintain variations in the quality factorbrought about by variations in said control voltage at values at whichthe stability and selectivity of said filter are substantially constant,and means coupled to the output circuit of said amplifer to derive anoutput voltage comprising said first signal voltage and said secondsignal voltage and having carrier frequency components relative tosideband frequency components greater than the carrier frequencycomponent of said first signal volt-, age.

2. An amplifier circuit arrangement, particularly for use in a receiverfor receiving signal voltages of carrier and sideband frequencies,comprising a resonant circuit for translating a first signal voltagehaving frequency components corresponding to said carrier andsaidlsideband frequencies, an amplifier having an input circuit and anoutput circuit, a filter circuit comprising an inductor having amagnetizable ferromagnetic core and being sharply tuned to a frequencycorresponding to said carrier frequency, means to couple said resonantcircuit and said filter circuit in cascade to the input circuit of saidamplifier to apply a signal voltage of fre-- quency corresponding tosaidcarrier and side band frequencies to said amplifier, means to couplesaid filter circuit to said amplifier to derive and apply to the inputcircuit of said amplifier a second signal voltage of frequencycorresponding to said carrier frequency to reduce the dampingof saidfilter, means coupled to said resonant circuit and the output circuit ofsaid amplifier to produce a frequency control voltage, means responsiveto said control voltage to vary the mag netization of said ferromagneticcore to tune said filter sharply to an intermediate frequencycorresponding to said carrier frequency, means coupling the outputcircuit of said amplifier to said,

filter circuit in regenerative feedback relationship to maintainvariations in the quality factor brought about by variations in saidcontrol voltage at values at which the stability and selectivity of saidfilter are substantially constant throughout the effective range of saidfilter, and meanscoupled to the output circuit of said amplifiento:derive an output voltage comprising 75, saidcarrier frequency, meanscouplingthe output;

said-first signal voltage and-said second signal voltage and havingcarrier frequency components relative to sideband frequency componentsgreater than the carrier frequency component first inductor and a secondinductor havinga magnetizable ferromagnetic core connected in series,means to couple said resonant circuit and said filter circuit in cascadeto the input circuit ofsaid amplifier to apply a signal voltage offrequency corresponding to said carrier and sideband frequencies to saidamplifier, means to cou-. ple said filtercircuit to said amplifier toderive and apply to the input circuit of said amplifiera second signalvoltage of frequency correspond-;-

ing to'sa-id carrier frequency to reduce the damping of said filter,means coupledto said resonant circuit and the output circuit of saidamplifier to produce a frequencycontrol voltage, means responsive tosaid controlvoltage to vary the :mag

netization of said ferromagnetic coreto tune said filter sharply to anintermediate frequency corresponding to said carrier frequency, meanscou-- pling the output circuit ofsaid amplifier to'the second inductorof said filter circuit in regenerative feedback relationship to maintainvariationsin the quality factor brought about by variations in saidcontrol voltage at values at which the stability and selectivity of saidfilter are-substantially constant over the effective range oi saidfilter, and means coupled to the output circuit of said amplifier toderive an output volt-- age comprising said first signal voltage andsaid second signal voltage and having carrier frequency componentsrelative to sideband frequency components greater than the carrier frequency component of said first signal voltage;

4. An amplifier circuit arrangement, particularly for use in a receiverfor receiving signal voltages of a carrier and sidebandfrequenciea'comprising a resonant circuit for translating a first signalvoltage having frequency components correspondingto said carrier andsaid sideband fre-, quencies, an amplifier having an input circuit and.v

an output circuit, a filter circuit sharply tuned to a frequencycorresponding to said carrier frequency, saidfilter circuit comprisinga'first inductor and a econd inductor having a magnetizableferromagnetic core connected in series, meansto couplesaid resonantcircuit and said filter circuit in cascadeto the inputcircuit of saidamplifier to apply a signal voltage of frequency corresponding to saidcarrier and sideband frequencies to said amplifier, means to couple saidfilter circuit to said amplifier to derive and apply to the inputcircuit of said amplifier a second si nal voltage of frequencycorresponding to said carrier frequency to reduce the damping of saidfilter, means coupled to said resonant circuit and the output circuit ofsaid amplifier to produce a frequency control voltage, means responsiveto said control voltage to vary the magnetization of a saidferromagnetic core to tune said filter sharply to an intermediatefrequency corresponding .to

circuit of said amplifier to the second inductor of said filter circuitin regenerative feedback relationship to maintain variations in thequality factor brought about by variations in said control voltage atvalues at which the stability and selectivity of said filter aresubstantially constant and said ferromagnetic core having dimensionalvalues and being constituted by material having magnetic values at whichthe series resistance of said filter circuit remains constant over theeffective range of said filter, and means coupled to'the-output circuitof said amplifier to derive an output voltage comprising said firstsignal voltage and said second signal voltage and having carrierfrequency components relative to sideband frequency components greaterthan the carrier frequency component of said first signal voltage.

5. An intermediate frequency amplifier circuit arrangement, particularlyfor use in a superheterodyne receiver for receiving signal voltages ofcarrier and sideband frequencies, comprising aresonant circuit fortranslating a first intermediate frequency signal voltage havingfrequency components corresponding to said carrier and said sidebandfrequencies, an electron discharge amplifier tube having a cathode, acontrol grid and an anode, a filter circuit comprising a first capacitorconnected in parallel with a first inductor and a second inductorconnected" in series, means to couple the first inductor of said filtercircuit to said resonant circuit and between the control grid andcathode of said amplifier tube to apply a signal voltage of frequencycorresponding to said carrier and sideband frequencies to said amplifiertube, means to couple said filter circuit to the cathode of saidamplifier tube to apply to the control grid of said amplifier tube asecond intermediate frequency signal voltage of frequency correspondingto said carrier frequency to reduce the damping of said filter circuit,means coupled to'the anode of said amplifier tube and to said resonantcircuit to derive a frequency control voltage, a third inductorelectromagnetically coupled by means of a ferromagnetic core to thesecond inductor of said filter circuit, means responsive to Saidfrequency control voltage to vary the current through said thirdinductor to 'vary the inductance of said second inductor to tune saidfilter sharply to an intermediate frequency corresponding to saidcarrier frequency, and means coupled to the anode of said amplifier tubeto derive an output voltage comprising said first intermediate frequencysignal voltage and said second intermediate frequency signal voltage andhaving carrier frequency components relative to sideband frequencycomponents greater than the carrier frequency component of said firstintermediate frequency signal voltage.

6. An intermediate frequency amplifier circuit arrangement, particularlyfor use in a superheterodyne receiver for receiving signal voltages ofcarrier and sideband frequencies, comprising a resonant circuit fortranslating a first intermediate frequency signal voltage havingfrequency components corresponding to said carrier and said sidebandfrequencies, a first electron discharge amplifier tube having a cathode,a control grid and an anode, a filter circuit comprising a firstcapacitor connected in parallel with a first inductor and a secondinductor connected in series, means to couple the first into saidcarrier and sdieband frequencies to said" ductor electromagneticallycoupled by means ofa ferromagnetic core to the second inductor of saidfilter circuit, a second electron discharge tube amplifier to apply saidfrequency control voltage to said third inductor automatically to varythe magnetization of said ferromagnetic core and to vary the inductancevalue of said second inductor to tune said filter sharply to anintermediate frequency corresponding to said carrier frequency, meanscoupling the anode circuit of said first electron discharge tube to saidfilter circuit in regenerative feedback relationship at a value at whichthe quality factor of said filter is substantially constant over theeffective range of said filter, and means coupled to the anode of saidfirst amplifier tube to derive an output voltage comprising said firstintermediate frequency signal voltage and said second intermediatefrequency signal voltage and having carrier frequency componentsrelative to sideband frequency components greater than the carrierfrequency component of said first intermediate frequency signal voltage.

7. An intermediate frequency amplifier circuit arrangement, particularlyfor use in a superheterodyne receiver for receiving signal voltages ofcarrier and sideband frequencies, comprising a resonant circuit fortranslating a first intermediate frequency signal voltage havingfrequency components corresponding to said carrier and said sidebandfrequencies, a first electron discharge amplifier tube having a cathode,a control grid and an anode, a filter circuit comprising a firstcapacitor connected in parallel with a first inductor and a secondinductor connected in series, said second inductor having a tappingthereon and a ferromagnetic core, means to couple the first inductor ofsaid filter circuit inductively to said resonant circuit and between thecontrol grid and cathode of said first amplifier tube to apply a signalvoltage of frequency corresponding to said carrier and sidebandfrequencies to said amplifier, means to couple said filter circuit tothe cathode of said amplifier tube to apply to the control grid of saidfirst amplifier tube a second intermediate frequency signal voltage offrequency corresponding to said carrier frequency to reduce the dampingof said filter circuit, a negative feedback resistor interposed betweenthe first inductor of said filter circuit and the cathode of said firstamplifier tube, a frequency responsive detector circuit coupled to theanode of said amplifier tube and to said resonant circuit to derive afrequency control voltage, a third inductor electromagnetically coupledto the second inductor of said filter circuit by means of saidferromagnetic core, a second electron discharge tube amplifier to applysaid frequency control voltage to said third inductor automatically tovary the magnetization of said ferromagnetic core and to vary theinductance value of said second inductor 9 to tune said filter sharplyto an intermediate frequency corresponding to said carrier frequency, asecond capacitor coupling the anode circuit of said first electrondischarge tube to said tapping on the second inductor of said filtercircuit to apply regenerative feedback to said filter at a value atwhich the quality factor of said filter is substantially constant overthe effective range of said filter, and means coupled to the anode ofsaid first amplifier tube to derive an output voltage comprising saidfirst intermediate frequency, signal voltage and said secondintermediate frequency signal voltage and having carrier frequencycomponents relative to sideband frequency components greater than thecarrier frequency component of said first intermediate frequency signalvoltage.

FOLKERT ALBERT DE GROOT.

REFERENQES CETED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

